Gas turbine plants consist essentially of a compressor, a burner and a gas turbine. In the compressor, air which has been sucked in is compressed before it is mixed with fuel in a combustion chamber in the downstream burner arranged in the compressor plenum and this mixture is burnt. The gas turbine located downstream of the combustion chamber then withdraws thermal energy from the combustion offgases formed and converts this thermal energy into mechanical energy. A generator connected to the gas turbine converts this mechanical energy into electric energy to generate power.
Current gas turbine plants, like other power-generating plants, have to generate very low pollutant emissions combined with maximum efficiency in all load ranges. The magnitude of the combustion temperature is limited by legally prescribed NOx values. The temperature in the combustion chamber, which forms the hot gas path between the burner and the gas turbine, is typically in an order of magnitude of from about 1300 to 1500 degrees Celsius. To be able to withstand these high temperatures, appropriate combustion chamber linings against such hot gas attack therefore have to be provided in order to protect the components and support structures enclosing the hot gas path.
Such heat shields can be either metallic or ceramic. In the case of gas turbine plants, ceramic materials are preferred because of the aggressive hot gases. Compared to metallic materials, such refractory ceramics have a higher temperature resistance and corrosion resistance and also a lower thermal conductivity. The materials on which such ceramic heat shields are based are high-α-alumina refractory ceramics which are, for example, produced by the pressure casting slip casting process described in DE 10 2008 011 820 A1.
Gas turbine plants now have to be adapted to the prevailing load conditions in a very short time. The highest stresses on the components and support structures of the gas turbine plant, i.e. including the heat shields, arise on quickly shutting down from base load. In such a case, the hot gas temperature can drop in a very short time by up to about 1000 kelvin. The thermal shock induced thereby in the heat shields makes it necessary for the refractory ceramic to have material properties which make possible a high strength combined with high thermal conductivity and do this at temperatures of up to about 1500 degrees Celsius and more. In addition, the refractory ceramics are required to have a high resistance to crack formation. EP 1 327 108 A1 discloses refractory ceramics which satisfy the abovementioned properties; for this purpose, the refractory ceramic has on average a different particle size distribution on the particularly stressed hot gas side than on the opposite, cooler combustion chamber wall side. However, such a particle size distribution has the disadvantage that additional internal stresses can be induced at the interfaces of the different pore size distributions, which can have an adverse effect on the passive reliability of the heat shields. Ceramics having an optimized homogeneous particle size distribution have a positive effect on the passive reliability of the overall system.